1. Field of the Invention
The present invention relates to a medical knife. In particular, it relates to a medical knife used for microsurgery such as ophthalmic surgery, cerebral surgery, or for surgery of blood vessels, etc.
2. Description of the Related Art
Conventionally, medical knives have been manufactured in the following manner. First, a wire rod of carbon steel, martensitic stainless steel, or precipitation hardened stainless steel 6 to 10 mm in diameter is used as a raw material. In the case of carbon steel and martensitic stainless steel, they are subjected to wire drawing into a wire rod approximately 1 mm in diameter. In the case of stainless steel, it is subjected to solution heat treatment between each wire drawing. The resulting processed wire rod is cut into pieces of an appropriate length, which are then subjected to annealing, press working into a flat shape, grinding to form blades, and quenching, making completed products. In the case of precipitation hardened stainless steel, after repeating wire drawing and solution heat treatment to make it approximately 1 mm in diameter, it is then cut into pieces of a pre-determined length. The pieces are then subjected to press working and grinding without being subjected to annealing, and then subjected to precipitation hardening treatment rather than quenching.
With such a manufacturing method, since wire rods are subjected to annealing before processing, processing is easily performed; however, quenching and precipitation hardening treatment tend to cause a crack, a chip or a break due to lack of toughness. Moreover, rust tends to develop due to the characteristics of the material, causing a problem of low corrosion resistance.
To solve such problems, Examined Japanese patent application laid-open No. Hei 1-11084 proposes a product manufacturing method for wire drawing a wire rod of austenitic stainless steel at a reduction ratio of 80% or greater and subjecting it to predetermined processing while keeping the temperature of the wire rod of steel at approximately 500° C. or less. The temperature of 500° C. or less is set, emanating from the fact that the hardness increased through work hardening decreases at 500° C. or greater. It is impossible for austenitic stainless steel to be subjected to quenching; instead, work hardening due to wire drawing is available. Use of austenitic stainless steel allows improvement in corrosion resistance. Moreover, since quenching is impossible, the problem of a quenching crack or chip cannot occur.
Japanese patent application laid-open No. Sho 59-87988 proposes use of composite stainless steel manufactured by generating a decarburized layer on the surface of martensitic stainless steel with high carbon density and high chromium density and then joining it with austenitic stainless steel. According to this proposal, a blade is formed in the martensitic stainless steel and then subjected to quenching to ensure sufficient sharpness, and high toughness of the austenitic stainless steel prevents a quenching crack or chip at the same time.
Examined Japanese patent application laid-open No. Hei 1-11084 is suitable for suture needles; however, it cannot provide sufficient hardness for necessary sharpness for knives. Meanwhile, the method using composite stainless described in Japanese patent application laid-open No. Sho 59-87988 is suitable for a fabrication of large cutting tools such as household cutting tools; however, it cannot be applied to a fabrication of small knives such as medical knives. In particular, it is very difficult to fabricate medical knives for microsurgery.